1. Field of the Invention
The present invention relates to load control systems for controlling the power delivered from an alternating-current (AC) power source to an electrical load, and more particularly, to a universal-voltage insulation detector for a recessed downlight fixture having a lighting load, such as a fluorescent lamp or a light-emitting diode (LED) light source.
2. Description of the Related Art
Downlight fixtures are typically mounted to a ceiling and are often recessed into the ceiling to provide an aesthetically pleasing appearance. FIG. 1 is a perspective view of a prior art recessed downlight fixture 10, which may be mounted to a ceiling, for example, in a drop ceiling panel 12. The downlight fixture 10 comprises a lighting load, e.g., a compact fluorescent lamp 14, which is installed in a housing 16. The lighting load could alternatively comprise an incandescent lamp or a light-emitting diode (LED) light source (i.e., an LED light engine). The downlight fixture 10 further comprises a junction box 18 for housing a load regulation device, such as an electronic ballast 20 (FIG. 2). The ballast 20 is electrically coupled to the fluorescent lamp 14 in the housing 16 and is required to drive the fluorescent lamp in order to properly illuminate the lamp. The junction box 18 comprises a conduit opening 22, which is adapted to be connected to an electrical conduit (not shown) for receipt of an alternating-current (AC) mains line voltage VAC from an AC power source 24 (FIG. 2). Alternatively, the downlight fixture 10 could comprise an LED driver (rather than the electronic ballast 20) if the lighting load is an LED light source.
In order to get a safety listing from a safety certification organization, such as Underwriters Laboratories (UL), lighting fixtures, such as the downlight fixture 10, must undergo a series of thermal tests. One test is the “abnormal test” in which the downlight fixture 10 (including the electronic ballast 20 and the controlled fluorescent lamp 12) is mounted in a simulated ceiling with insulation placed on top of the downlight fixture. The temperature on the outer surface of the downlight fixture 10 either must not exceed a temperature limit during the test, or, if the temperature of the downlight fixture exceeds the temperature limit, the lighting load must be turned off within three hours of exceeding the temperature limit.
Accordingly, the prior art downlight fixture 10 may include an insulation detector 30 that is electrically coupled to the electronic ballast 20 and operates to cut off the AC mains line voltage VAC from the electronic ballast 20 and the fluorescent lamp 12 when insulation is present around the insulation detector, such that the downlight fixture is able to pass the abnormal test. The insulation detector 30 is mounted to an exterior surface of the housing 16 or the junction box 18 (e.g., in a conduit opening as shown in FIG. 1), such that the insulation detector protrudes into the space around the fixture 10, i.e., into the insulation surrounding the insulation detector.
FIG. 2 is a simplified schematic diagram of the downlight fixture 10 showing the insulation detector 30 in greater detail. The insulation detector 30 comprises three connections: a hot connection H electrically coupled to the hot side of the AC power source 24, a neutral connection N electrically coupled to the neutral side of the AC power source, and a switched hot terminal SH coupled to the electronic ballast 20. The insulation detector 30 is coupled in series electrical connection between the AC power source 24 and the electronic ballast 20, and is able to disconnect the AC mains line voltage VAC from the ballast.
The insulation detector 30 comprises a thermal cut-off (TCO) switch 32 (i.e., a bimetallic switch) and a resistor 34. The TCO switch 32 and the resistor 34 are both contained within an elongated thermally-conductive enclosure 36 (as shown in FIG. 1). The resistor 34 operates as a heat source when the TCO switch 32 is closed and the AC mains line voltage VAC is coupled across the hot and neutral terminals H, N. The cut-off temperature of the TCO switch 32 is chosen such that the TCO switch remains closed when the downlight fixture 10 is mounted to a ceiling without insulation surrounding the downlight fixture. In other words, the heat generated by the resistor 34 dissipates from the insulation detector 30 into the surrounding air without causing the TCO switch 32 to open. However, if the downlight fixture 10 is surrounded by insulation (e.g., during the abnormal test), the heat generated by the resistor 34 is not able to dissipate into the insulation and the temperature inside the insulation detector 30 increases, such that the TCO switch 32 opens, thus disconnecting the electronic ballast 20 from the AC power source 24. An example of the prior art insulation detector 30 is described in greater detail in U.S. Pat. No. 6,921,884, issued Jul. 26, 2005, entitled SELF-HEATING THERMAL PROTECTOR, the entire disclosure of which is hereby incorporated by reference.
Typical electronic ballasts and LED drivers are able to operate across a range of AC mains line voltages (e.g., from approximately 120 V to 277 V). However, prior art insulation detectors, such as the one described in the '884 patent, are only rated to operate with a specific AC mains line voltage (e.g., at either 120 V or 277 V). As a result, lighting fixture manufacturers must manufacture and stock separate downlight fixtures for both of the rated voltages of the insulation detectors 30 even though the electronic ballasts and LED driver operates across a range of AC mains lines voltages. This, of course, leads to additional stock keeping units (SKUs) and increased inventory cost for the lighting fixture manufacturer.
Accordingly, there is a need for an insulation detector that is able to operate at a plurality of different AC mains line voltages.